The present invention generally relates to methods and apparatus whereby the speed of a moving entity is measured by a magnetic pickup coil subject to variable magnetic fields as ferromagnetic discontinuities on the moving entity move past the magnetic pickup coil and, more specifically, to apparatus and methods for the generation of a clean square-wave pulse train from a noisy signal received from the pickup coil. The ferromagnetic discontinuities generally have no magnetism of their own. They are detected by a passive magnetic sensor which includes a permanent magnet and the magnetic pickup coil.
When a ferromagnetic discontinuity on the moving entity approaches or recedes from the passive magnetic sensor, magnetic flux inside the coil changes and, by Gauss""s law, a variable electromotive force (emf) or voltage is generated in the coil. Systems for measuring the speed of a rotating or translating entity are known wherein ferromagnetic discontinuities, which generally have no magnetism of their own, are disposed on the moveable entity, spaced apart in a direction of movement of the entity. One or more passive magnetic sensors are placed adjacent the ferromagnetic discontinuities so that movement of the entity causes electrical pulses to be induced in the sensors as the ferromagnetic discontinuities move past the sensor(s).
For measuring rotary motion, the discontinuities are generally formed in a circular array. For example, a ferromagnetic gear may be placed on a shaft, and the teeth of the gear constitute the ferromagnetic discontinuities. Alternatively, slots or flutes may be formed in a ferromagnetic shaft to delineate the required ferromagnetic discontinuities, which are uncut portions of the shaft between the slots or flutes. In either case, a passive magnetic detector is placed adjacent the ferromagnetic discontinuities so that, as the shaft rotates, the ferromagnetic discontinuities cause variable magnetic flux inside the coil of the detector, and hence generate variable emf""s in the coil.
For measuring linear motion, the ferromagnetic discontinuities are generally formed as parallel ridges spaced apart laterally in the direction of motion. The ridges, preferably, lie perpendicular to the direction of motion. A type of gear known as a xe2x80x9crackxe2x80x9d may be employed for this purpose.
Since the emf generated by such a coil depends on the rate of change of magnetic flux, such a coil generates a signal that alternates between negative and positive values. If the ferromagnetic discontinuities are uniformly sized and spaced, the emf from the coil will comprise periodic alternating positive and negative segments. It is known to generate a train of clean square-wave pulses from the coil emf. A zero crossing detector is employed for this purpose. When the coil emf crosses zero in the positive direction, the output of the zero crossing detector is set to one level. When the coil emf crosses zero in the negative direction, the output of the zero crossing detector is set to another level. For example, the output may go high for a positive crossing and low for a negative crossing.
In order to reduce the sensitivity of the zero crossing detector to noise, it is known to employ an upper threshold to indicate zero crossing in the positive direction, and a lower threshold to indicate zero crossing in the negative direction. In prior art systems for measuring speed of rotating or translating entities, these thresholds are generally set at fixed, constant values.
It is noted that the signal generated by the pickup coil increases linearly in strength with the speed of the moving entity. Therefore, in prior art systems, as the velocity of the moving entity increases, the signal gets stronger, and the threshold becomes a smaller and smaller fraction of the signal strength. This is inconsistent with a rule of thumb known in the art, namely, that the thresholds should have a magnitude of about one fifth to one eighth of the peak signal strength.
Two prior art patents teach pulse detection systems employing zero crossing detectors with variable thresholds. Both of these patents are for computer disk drive data reading, not for sensing speed of a translating or a rotating body, and neither patent adjusts the threshold as a function of measured signal strength in real time.
U.S. Pat. No. 5,287,227 teaches a manufacturing system in which several points are tested on the disk surface during disk drive manufacture. Thresholds for the zero crossing detector that will later be used to process a signal from the disk reading head are determined based on the quality of the surface points for accepting and retaining a digital test signal. Extrapolation is done for points in between the tested points. The thresholds are stored in memory and written onto the disk during manufacture. Later, when the disk is started up, the thresholds are read from the disk and stored in memory. Then, when data is read from the disk, thresholds are obtained from memory and employed in the zero crossing detector. This invention does not adjust the thresholds in accordance with an ambient noise level or the strength of the signal actually obtained by the read head during reading of the disk.
U.S. Pat. No. 5,150,050 teaches a manufacturing system in which tests are made at points on the disk surface during disk drive manufacture. Thresholds for a zero crossing detector are varied to determine whether a spot on the disk can reliably be written to and read from. If no such threshold can be found for a spot on the disk, the location of the spot is stored and is written onto the disk. Later, when the disk is in use, the bad spots on the disk are not used for storing information. This patent also teaches a system whereby, when a disk data read fails, the threshold is changed by a predetermined amount and a data re-read attempted, in order to recover the data.
It is noted that the use of a zero crossing detector in the references cited differs greatly from the use in a speed sensor employing a pickup coil. The mechanism for detecting a pulse from the pickup head of a disk drive is not a zero crossing detector, but a peak detector. The variable-threshold zero-crossing detector merely enables the peak detector, such that only the first peak after a zero-crossing is counted. The peak detector is mechanized via a zero-crossing detector with a fixed zero-volt threshold acting on a time derivative of the sensed signal.
It is further noted that the references cited do not dynamically compensate for degradation of either the write head or the read head, or a change in signal strength from any other cause, such as height of the heads above the disk, or special misalignments. They furthermore do not dynamically compensate for variations in electrical noise. U.S. Pat. No. 5,287,227 does not compensate at all, while U.S. Pat. No. 5,150,050 compensates only after a data read has failed, adjusting the threshold by pre-determined amounts, rather than constantly adjusting the thresholds in real time to prevent the data read failure in the first place.
As can be seen, there clearly is a need for a speed sensing system employing a passive magnetic sensor which detects pulses of a signal from the sensor by a zero crossing detector having a variable threshold. Such a system would allow use of detectors with weak outputs at low speeds (which are less failure prone due to larger wire sizes) without undue sensitivity to noise at higher speeds, would compensate for degradation of the sensor magnet, variations in spacing of the detector to the ferromagnetic discontinuities which cause the signal, and would compensate for detector failure modes and for ambient noise.
In one aspect of the present invention, a turbofan engine comprises a gas turbine engine and a bypass fan driven by the gas turbine engine. At least one shaft of the turbofan engine has a circular array of ferromagnetic discontinuities either formed as a portion of the shaft or attached to the shaft to rotate with the shaft. A passive magnetic sensor is positioned adjacent the circular array of ferromagnetic discontinuities, the passive magnetic sensor having at least one sensor coil whereby a sensor signal is generated in the sensor coil(s) by movement of the ferromagnetic discontinuities past the passive magnetic sensor. The sensor signal has pulses indicative of the speed of the shaft. A filter stage has an input connected to at least one of the sensor coils, the filter stage removing electrical noise from the sensor signal. A signal strength sensing circuit is connected to either the sensor coil or an output of the filter stage (typically it would only be connected to the output of the filter stage, as the zero-crossing detector acts on the output of the filter stage and it is the ratio of threshold-to-signal into the zero-crossing detector that we are trying to maintain in the 5:1 to 8:1 range. If it were connected to the input of the filter stage, the ratio would drop at higher speeds as the filter attenuates the signal). The signal strength sensing circuit generates a signal strength indicating signal based on the strength of the sensor signal. A zero crossing detector is connected to the output of the filter stage to receive the sensor signal, and is connected to the signal strength sensing circuit to receive the signal strength indicating signal. The zero crossing detector generates a clean square-wave pulse train from the sensor signal. It has an upper threshold to test for zero crossing in a positive direction and a lower threshold to test for zero crossing in a negative direction, at least one of the upper threshold and the lower threshold depending on the signal strength indicating signal. The rate or frequency of the clean square-wave pulse train is indicative of the rotary speed of the shaft and hence a speed of the turbofan engine.
In another aspect, the present invention is a speed sensing system for sensing speed of a moveable entity. The speed sensing system includes a plurality of ferromagnetic discontinuities either attached to or formed as a portion of the moveable entity. The ferromagnetic discontinuities are spaced apart in a direction of movement of the moveable entity. The speed sensing system has a passive magnetic sensor including at least one sensor coil, the passive magnetic sensor being positioned adjacent the ferromagnetic discontinuities so that movement of the moveable entity causes the ferromagnetic discontinuities to move past the passive magnetic sensor. The ferromagnetic discontinuities induce a sensor signal in the sensor coil(s). A filter stage is attached to at least one sensor coil, the filter stage removing electrical noise from the sensor signal. A signal strength sensing circuit is connected to either the sensor coil or to an output of the filter stage to receive the sensor signal. The signal strength sensing circuit produces a variable threshold signal. The variable threshold signal is a substantially monotonically non-decreasing function of the strength of the sensor signal. Generally, it increases with the strength of the sensor signal, but may be limited to a predetermined maximum. A variable threshold zero crossing detector is connected to the output of the filter stage to receive the sensor signal and to an output of the signal strength sensing circuit to receive the variable threshold signal. The variable threshold zero crossing detector has an upper threshold to test for zero crossing in a positive direction and a lower threshold to test for zero crossing in a negative direction. Either or both of the thresholds are dependent on the variable threshold signal. The variable threshold zero crossing detector generates a clean square-wave pulse train indicative of positive and negative zero crossings. The speed sensing system also has a speed indicating circuit generating a speed signal indicative of either the number of pulses per unit time, or the elapsed time between pulses.
In another aspect of the present invention, an electronic circuit measures a pulse frequency and/or a pulse period of pulses in a sensor signal originating in a sensor coil, the pulses being zero crossing pulses. The circuit includes a filter stage having an input connected to the sensor coil, the filter stage being for removing electrical noise from the sensor signal. The circuit also has a signal strength sensing circuit connected to receive the sensor signal, the signal strength sensing circuit producing a variable threshold signal, which is a substantially monotonically non-decreasing function of strength of the sensor signal. A variable threshold zero crossing detector is connected to receive the sensor signal and the variable threshold signal. The variable threshold zero crossing detector has an upper threshold to test for zero crossing in a positive direction and a lower threshold to test for zero crossing in a negative direction. One or both of the thresholds are dependent on the variable threshold signal. The variable threshold zero crossing detector generates a clean square-wave pulse train indicative of positive and negative zero crossings. The circuit also includes an output circuit generating a signal indicative of the pulse frequency and/or the pulse period.
In an additional aspect of the present invention, a microprocessor-controlled circuit measures a pulse frequency and/or a pulse period of zero crossing pulses in a sensor signal originating in a sensor coil. The microprocessor-controlled circuit includes a filter stage having an input connected to the sensor coil, the filter stage for removing electrical noise from the sensor signal. The microprocessor-controlled circuit also has a microprocessor-enabled zero crossing detector connected to an output of the filter stage, the microprocessor-enabled zero crossing detector producing a clean square-wave pulse train from the sensor signal, the zero crossing detector employing an upper threshold indicative of zero crossing in a positive direction and a lower threshold indicative of zero crossing in a negative direction. The microprocessor, preferably, receives a digital indication of the signal strength, typically via an analog-to-digital conversion of the signal strength measurement. The microprocessor then determines either or both the upper threshold and the lower threshold based on a strength of the sensor signal, and provides a signal indicative of the desired threshold to the zero-crossing detector via a digital-to-analog converter. The absolute magnitude of either or both of the thresholds is a monotonic non-decreasing function of the strength of the sensor signal. A speed indicating circuit is connected to receive the clean square-wave pulse train from the zero crossing detector, the speed indicating circuit generating an output signal indicative of either a pulse frequency or a pulse period of the sensor signal, or both.
In a further aspect, the invention is a method of measuring at least one of a pulse frequency and a pulse period of zero crossing pulses originating in a sensor coil. The method includes producing a variable threshold signal based on a strength of the sensor signal, the variable threshold signal being a substantially monotonically non-decreasing function of the strength of the sensor signal. A clean square-wave pulse train having a of uniform height is produced from the sensor signal by a zero crossing detector. The zero crossing detector has an upper threshold to detect zero crossings in the positive direction and a lower threshold to detect zero crossings in a negative direction. Either or both of the upper threshold and the lower threshold are dependent on the variable threshold signal. The method further includes processing the clean square-wave pulse train to generate a signal indicative of either the pulse frequency or the pulse period.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.